Several golden age characters first appeared in the context of the silver age world of Earth-One, wherein it was thought that Superboy was that world's first hero approximately twenty-some years prior to the presently published tales. For instance, the Guardian appeared with his Newsboy Legion aiding that world's Jimmy Olsen, Manhunter teamed up on one occasion with the modern Batman, and Air Wave was identified as the cousin of Hal Jordan, the Green Lantern. Both were Jordans, after all, so why not tie them together?
Then the decidedly different All-Star Squadron was published, and all these characters were members of that team back in the 1940's when their tales were published. Further, the series Who's Who which was published concurrent with the Crisis on Infinite Earths listed each character's history (as well as that of Zatara, Sargon, and Commander Steel) from their joining the All-Stars to their modern-day meetings of Earth-One's heroes. This was then explained in an All-Star letter column as individuals who migrated from Earths Two to One (and this was further solidified during a team-up of the Justice League and Justice Society by a comment Flash made to teammate Power Girl when she met Sargon)
Yet how could Air Wave be cousins with Mr. Jordan? Further, how could Jim Harper, the Guardian, be the uncle of Roy Harper aka Speedy? We can explain this quite simply. When he moved to Earth-One, Larry Jordan sought out familial connections. Instead, he met Helen Jordan and fell in love with her. She was the actual cousin of Hal, Larry was but an in-law. And the Guardian? Remember there were TWO Roy Harpers who would become Green Arrow's sidekick Speedy. While uncle to the Earth-Two version, Jim Harper himself sought out family when moving to Earth-One and was the "long lost" uncle of the orphaned lad. This is not unheard of, for Earth-Two's Huntress referred to her father's Earth-One counterpart Bruce Wayne as "Uncle Bruce". Earth-One's Superman and Earth-Two's Power Girl affectionately called one another cousins on occasion.
In fact, Project Cadmus on Earth-One mirrored Project M of Earth-Two, which was a top secret government agency operating in America during the early days of World War II. Project M (for Monster) was founded by Professor Mazursky, who like Cadmus founder Dabney Donovan, was a "mad scientist" type and thus very unethical in his approach towards science. Both Projects specialized in creating clones and synthetic monsters, with some being successful such as Dubbilex the DnAlien (whose Monster doppleganger never successfully awoke from its hatching chamber) as well as Miss America (whose Earth-One twin Element Girl may have been given her powers by an earlier incarnation of Cadmus. In fact, the latter would herself be another migrant to a third parallel planet known as Earth-X alongside the Freedom Fighters where she perished. Other creations of Mazursky and his colleague Chuck Grayson, who was the assistant to robotics scientist Robert "Robotman" Crane, were the Creature Commandos and G.I. Robot. The fact that both Grayson and Miss America were a part of Project M solidifies that it was the Earth-Two analogue to Project Cadmus. And Crane would also coincidentally be a migrant to Earth-One, establishing his alter ego of the Paul Dennis of that world before returning to his native world and universe with his mind now housed in Grayson's body! More migrants tied to these individuals will be revealed henceforth...